1. Field of the Invention
The present invention relates generally to implantable prostheses for replacing human skeletal joints, and relates more particularly to an acetabular cup component of a total hip prosthesis.
2. Background Information
An implantable total hip prosthesis includes a femoral component and an acetabular cup component. The femoral component, typically made of a bio-compatible metal such as titanium, titanium alloy or cobalt chrome alloy, has a distal stem and a proximal spherical head. As used herein, the words proximal and distal are terms of reference that indicate a particular portion of a prosthesis component according to the relative disposition of the portion when the component is implanted. "Proximal" indicates that portion of a component nearest to the torso, whereas "distal" indicates that portion of the component farthest from the torso. The distal stem is configured to be received and fixed within the medullary canal of a femur. The femur is prepared to receive the distal stem by resecting the natural head and neck, and reaming the exposed proximal medullary canal. The proximal end of the femoral component has a neck and attached spherical head that, supported by the distal stem anchored in the medullary canal, extend from the resected proximal end of the femur to replace the natural neck and head of the proximal femur. The proximal spherical head may be integral with the neck and stem of the femoral component, or may be removably attachable to the neck by way of a locking taper connection, sometimes known as a Morse taper. The acetabular cup component is configured to be received and fixed within the acetabulum of a pelvis. The pelvis is prepared to receive the acetabular cup by reaming a concavity in the acetabular bone. The acetabular cup component typically has an outer surface conforming to the concavity reamed in the acetabular bone of the pelvis, and an inner bearing cavity for receiving the head of the femoral component. The head articulates in the bearing cavity as a ball and socket to restore motion to a diseased or damaged hip joint.
One known type of acetabular cup involves an acetabular shell made of a bio-compatible metal such as titanium or a titanium alloy, and a bearing insert made of a bio-compatible polymer such as ultra-high molecular weight polyethylene. The acetabular shell is shaped generally as a hemispherical cup having a dome, or apex, at a proximal end and an annular rim at a distal end. Between the dome and rim, the acetabular shell comprises a shell wall defined by a generally convex proximal surface and a generally concave distal surface spaced from the proximal surface. The concave distal surface defines a shell cavity having an opening at the rim of the cup for receiving the bearing insert. The bearing insert has a generally convex proximal surface configured to be received and fixed within the acetabular shell in generally congruent engagement with the concave distal surface of the shell wall. The bearing insert also has a bearing cavity that opens distally for receiving the head of the femoral component. The bearing cavity is defined by a generally spherical concave bearing surface having a radius similar to that of the femoral head component. The concave bearing surface articulates against the surface of the spherical femoral head component.
Acetabular shells of the type described can be affixed to the acetabular bone by bone screws or bone cement. If bone screws are elected, the screws are driven into the bone through the screw holes before the bearing insert is placed into the shell. The shell also can be affixed by a combination of bone screws and bone cement. The acetabular shell can be provided with more screw holes than typically would be used by the implanting physician. This provides a selection of sites for placement of the bone screws, as may be dictated by the condition of the patient's pelvic bone or by the physician's preference. Some of the provided screw holes may receive a screw while others do not. For reasons explained below, it is desirable to provide means for occluding those screw holes that will not receive a screw.
Commonly, acetabular shells of the type described also include a dome hole at the apex. A typical dome hole is coaxially aligned with the axis of symmetry of the acetabular shell and extends through the shell wall from the concave distal surface to the convex proximal surface of the acetabular shell. Often, the dome hole is internally threaded or otherwise configured for receiving an instrument for holding and positioning the acetabular shell during implantation. Also, many physicians use the dome hole to obtain visual or tactile access to the reamed acetabular bone during implantation of the acetabular shell. Such access allows the physician to confirm that the acetabular shell is fully seated in engagement with the reamed bony surface of the acetabulum. By looking through the dome hole from the distal side of the shell, or by probing through the dome hole from the distal side of the shell with an elongate, bent, pointed instrument, the physician can discern whether space exists between the proximal opening of the dome hole and the adjacent acetabular bone. If such space exists, then the acetabular shell is not fully seated and the physician must make appropriate corrections to fully seat the acetabular shell in close engagement with the bone. As with the screw holes, for reasons explained below, it is also desirable to provide means for occluding the dome hole.
The bearing insert is usually designed to be received within the acetabular shell in nonarticulating relative relationship. Nevertheless, a small amount of unintended relative motion is believed to occur between the bearing insert and the acetabular shell in response to the varying load borne by the acetabular cup during use. Such small relative motion, or micro-motion, may result in wear at the interface between the bearing insert and acetabular shell that generates fine polyethylene or metal debris. According to some hypotheses, such debris can migrate out of the acetabular cup and contact bone, possibly resulting in osteolysis, which ultimately can lead to bone resorption and possible loosening of the acetabular prosthesis. One apparent pathway for the migration of debris out of the acetabular shell is through open screw holes. Another apparent pathway is through an open dome hole.
Various proposals for selectively occluding screw holes and dome holes in prosthetic acetabular shells are known in the art. The present invention is particularly directed to providing a means for occluding a dome hole while preserving the shell positioning and seating confirmation functions of the dome hole. It would be desirable to provide an acetabular shell, designed for use with a bearing insert, having a dome hole that can receive a holding and positioning instrument and that allows the physician to confirm that the acetabular shell has been fully seated against bone. The present invention provides these and other desirable advantages.